As known, an optical cable typically comprises an optical core and an external sheath enclosing the optical core. The external sheath is typically made of a polymeric material and has the primary function of protecting the optical core from the mechanical stresses.
The optical core of the cable typically comprises one or more optical modules, the term “optical module” indicating a fiber bundle—comprising one or more optical fibers—and a retaining element (e.g. a buffer tube or a tape) surrounding the optical fibers. The retaining element carries out the function of retaining the optical fibers and, optionally, protecting them against mechanical stresses.
An optical fiber is typically constituted by a glass core, a glass cladding and a single or dual polymeric coating layer.
The optical fibers may be arranged in various ways within the cable.
For instance, in the so-called “central loose tube cables” (briefly, CLT cables) a single optical unit comprising optical fibers loosely arranged within a buffer tube is enclosed by the external sheath.
U.S. Pat. No. 6,178,278 describes a CLT cable, wherein the buffer tube is a dry-loose tube and has a fiberglass yarn matrix arranged about it. An external jacket is arranged about the fiberglass yarn matrix. Ripcords are also provided between the dry-loose tube and the jacket.
The so-called “multi loose tube cables” (briefly, MLT cables) typically comprise multiple optical modules (e.g. 3, 4 or 6 optical modules) stranded about a central strength member. A binder may also be provided around the optical modules for retaining them. An external sheath surrounds all the optical modules.
U.S. Pat. No. 6,424,769 describes a MLT cable comprising multiple optical modules arranged within an external sheath. According to an embodiment, a first array of six optical modules is arranged around a central core, while a second array of twelve optical modules is arranged around the first array of six optical modules, so that the first and second array of optical modules are coaxial.
In optical cables, the identification of each single optical fiber may be determined by a color coding. The international standard ANSI/EIA/TIA-598 “Optical Fiber Color Coding” in particular defines twelve individual colors plus twelve individual colors with tracers (e.g. ring marks every 100 mm along the fibers). Hence, in order to allow individual identification of each single fiber comprised in an optical module, the optical module may comprise at most 12 optical fibers (if no tracers are used) or 24 optical fibers (if tracers are used).
In order to increase the fiber count of an optical module (and hence of the whole cable) beyond such limits, it is known grouping the optical fibers in several wrapped bundles.
U.S. Pat. No. 6,321,012 describes an optical cable wherein the optical fibers are divided in groups (e.g. four groups), each group being wrapped by a water-swellable material tape having a unique color. Each optical fiber is identified by a unique combination of tape color and fiber color.